Unreal 227 has an option to save inventory of players on each map change that allows inventory to travel from level to level (see Unreal.ini):

[Engine.GameEngine]bServerSaveInventory=True

The inventory is saved to InventoryData.ini. This is not the same as saving/loading game in single-player game (when you can save/load the progress at any moment), but it lets you continue from the start of the most recent loaded level, after the server has been shut down and launched again.

For example, you can finish all maps from Vortex2 to Dark inclusively, then, when Harobed is loaded, shut the server down, then run the server again, choose map Harobed, join the game with the same nicknames as before, and continue your cooperative journey with the same set of inventory that you had when finishing Dark.